Each year, 9.6% of infants, 13 million worldwide, are born with low birth weight with a mortality rate 20 times higher than in normal babies. Most of these preterm infants as well as some term babies have abnormal or no pulmonary surfactant, a lipid-protein mixture secreted by alveolar type II epithelial (AEII) cells. When surfactant functio is abnormal, the lung partially collapses hindering gas exchange, a severe condition that can lead to death. Patients suffering from surfactant deficiency invariably require mechanical ventilation. During the last decade, variable ventilation (VV) emerged and proved to be superior to conventional ventilation (CV). In VV, tidal volumes (VT) are varied on a breath-by-breath basis such that VT is drawn from a distribution optimized to achieve best alveolar recruitment. Previously, we found that in normal guinea pigs, VV increased surfactant concentration at the air-liquid interface due to increased surfactant release that we directly proved by stretching primary AEII cells in culture using a variable stretch pattern. Since mechanical stretch is perhaps the most potent stimulant for surfactant release by AEII cells, the stretch pattern applied to AEII cells in vivo might have a crucial importance in the outcome of ventilation of infants. Hence, we hypothesize that during conditions of surfactant deficiency, the application of VV will lead to enhanced surfactant production and secretion with improved lung function and gas exchange compared to CV. To test this hypothesis, we set up 3 aims: 1) To study the signaling pathways of variable stretch-induced surfactant metabolism including gene regulation and release by AEII cells in culture isolated from healthy baby rabbits and surfactant deficient preterm rabbits. 2) To determine the optimal conditions under which VV enhances surfactant gene upregulation and release by AEII cells in healthy baby rabbits and surfactant deficient preterm rabbits and lambs. 3) To test in baby rabbits and lambs the effectiveness of a novel mechanical assay which non-invasively monitors the organ level functionality of the surfactant system. Studies will be carried out at the cell, tissue and organ level to reveal how VV enhances surfactant secretion through regulating specific fusion pore proteins. If the results confirm our hypothesis, the implications are truly important with significant impact on neonatal care. For example, we will uncover the cellular mechanism of variable stretch-induced surfactant production and release. The treatment will not require exogenous surfactant therapy. Rather, using a simple mechanical perturbation (tuned variability in VT), we will stimulate AEII cells in vivo to generate more surfactant, a procedure we call endogenous surfactant therapy. The method is inherently safe as there is no radiation, chemical treatment or any other harm associated with moderately varying VT. Thus, we expect to start translating the technology to clinical practice toward the end of the award period. In conclusion, this simple mechanical intervention may reduce the morbidity and mortality associated with surfactant deficiency in infants and children by appropriately steering the body's endogenous response.